The present invention relates to an average frequency measuring apparatus which measures the frequency of an input signal a plurality of times and averages the measured values to thereby obtain the input frequency with higher accuracy.
A method that has been used for measuring the frequency of an input signal is to gate the input signal by a gate signal of a fixed duration, to count the number of cycles in the gated output as a pulse for each cycle and to obtain the input frequency from the count value. In this case, there is a possibility that the count value varies by one count from another count value for the same input frequency due to the relative phase relation between the input signal and the gate signal, resulting in what is called a quantizing error. With a view to minimizing the influence of such a quantizing error, a method is employed in which the frequency of the input signal is measured a plurality of times and the average of the measured values is regarded as the correct frequency of the input signal. With such a method, however, if the relative phase relation between the input signal and the gate signal for each measurement is constant, then the averaging of the measured values loses its meaning. How to vary the relative phases of the input signal and the gate signal at random from this viewpoint is set forth in U.S. Pat. No. 3,938,042 entitled "Measurement Averaging Counting Apparatus Employing a Randomly Phase Modulated Time Base to Improve Counting Resolution" issued on Feb. 10, 1976. In the apparatus set forth in this patent, a clock signal is phase modulated by random noise and the phase modulated clock signal is frequency multiplied to obtain a clock signal for counting. But this apparatus is defective in that it necessitates the use of a relatively complex modulator for the phase modulation and a frequency multiplier. Moreover, this apparatus has the defect of low measurement accuracy because the phase of the clock signal undergoes variations even during the counting operation.
On the other hand, there are some occasions when it is desired to measure the carrier frequency of a signal whose carrier signal has been amplitude modulated 100% by a pulse signal, that is, what is called a burst signal having a discontinuous carrier signal. In the measurement of the carrier frequency of such a burst signal, even if the carrier frequency is measured by counting the number of carrier cycles in one burst signal, no satisfactory accuracy can be obtained when the burst signal width is small. To avoid this, the measurement is carried out by counting the number of carrier cycles for a plurality of burst signals; in this case, however, when the measurement period is made constant, the measurement accuracy varies with the length of each burst signal.
Further, from the viewpoint of increasing the accuracy in the measurement of the carrier frequency of the burst signal, a count is taken of the carrier in a plurality of burst signals. In such a case, in order to lessen the influence of the quantizing error present for each burst signal, the phase of the gate signal is changed at random relative to each burst signal. However, if the width of each gate signal is made small so that the gate signal should not extend outside the corresponding burst signal, the time available for counting the carrier is reduced. Therefore, to improve the measurement accuracy, the overall measurement time must be lengthened.